NASA to Explore a "Secret Layer" of the Sun

Sept.
5, 2008: Next April, for a grand total of 8 minutes,
NASA astronomers are going to glimpse a secret layer of the
sun.

Researchers
call it "the transition region." It is a place in
the sun's atmosphere, about 5000 km above the stellar surface,
where magnetic fields overwhelm the pressure of matter and
seize control of the sun's gases. It's where solar flares
explode, where coronal mass ejections begin their journey
to Earth, where the solar wind is mysteriously accelerated
to a million mph.

It
is, in short, the birthplace of space weather.

Researchers
hope it is about to yield its secrets.

Right:
Not far above the surface of the sun lies the "transition
region" where magnetic fields seize control of solar
gases. Photo credit: NASA/TRACE.

"Early
next year, we're going to launch an experimental telescope that
can measure vector magnetic fields in the transition region,"
explains Jonathan Cirtain of the Marshall Space Flight Center
(MSFC). Previous studies have measured these fields above
and below the transition region—but never inside it.
"We hope to be the first."

The
name of the telescope is SUMI, short for Solar Ultraviolet
Magnetograph Investigation. It was developed by astronomers
and engineers at the MSFC and is currently scheduled for launch
from White Sands, New Mexico, in April 2009.

SUMI
works by means of "Zeeman splitting." Dutch physicist
Pieter Zeeman discovered the effect in the 19th century. When
a glass tube filled with incandescent gas is dipped into a
magnetic field, spectral lines emitted by the gas get split
into two slightly different colors—the stronger the field,
the bigger the splitting. The same thing happens on the sun.
Here, for instance, are some spectral lines from gaseous iron
being split by the magnetic field of a sunspot:

By
measuring the gap, astronomers estimate the strength of the
sunspot's magnetic field. Furthermore, by measuring the polarization
of the split line, astronomers can figure out the direction
of the magnetic field. Strength + direction = everything you
ever wanted to know about a magnetic field!

This
trick has been applied to thousands of sunspots on the solar
surface, but never to the transition region just a short distance
above.

Why
not?

"Just
bad luck, really," says Cirtain. "Gas in the transition
region doesn't produce many strong spectral lines that we
can see at visible wavelengths." It does, however, produce
lines at UV wavelengths invisible from Earth's surface.

"That's
why we have to leave Earth."

SUMI
will blast off inside the nose cone of a Black Brant rocket
on a sub-orbital flight that takes it to an altitude of 300
km. "We'll be above more than 99.99% of Earth's atmosphere,"
says Cirtain. About 68 seconds into the flight, payload doors
will open, affording SUMI a crystal-clear view of the UV sun.
"From that moment, we've only got 8 minutes to work with.
We'll target an active region and start taking data."

Right:
A Black Brant sounding rocket of the type that will carry
SUMI above Earth's atmosphere.

SUMI's
"vector magnetograph" is tuned to study a pair of
spectral lines: one from triply-ionized carbon (CIV) at 155
nanometers and a second from singly-ionized magnesium (MgII)
at 280 nanometers. "There's nothing special about those
ions," notes Cirtain. "They just happen to produce
the best and brightest lines at temperatures and densities
typical of the transition region."

Cirtain
anticipates how it will feel to have his precious instrument
hurtling 300 km above Earth at 5,000 mph: "Eight minutes
of terror." He'll
start breathing again when the payload doors close and SUMI
begins its descent back to Earth. Cirtain ticks off the stages:
"Reentry into the atmosphere. Open parachutes. Landing
back at White Sands. Recovery."

The
short flight probably won't lead to immediate breakthroughs.
"But it will demonstrate the SUMI concept and show us
if it's going to work." A successful flight would lead
to more flights and eventually to a SUMI-style magnetograph
permanently installed on a space telescope.